Electronic devices integrated with a tire can provide functions such as identification and tracking during manufacture, distribution, and use. These electronics can also provide for measurement of physical parameters such as pressure and temperature during use of the tire. Many systems utilize radio frequency communication between the tire and an external monitoring or interrogating device. A radio frequency communication link requires one or more antennas.
There are available systems that mount to a surface of the tire or the wheel, or are incorporated in the tire inflation valve. An electronic device and antenna attached directly to a surface of the tire or embedded in a tire structure is desirable as providing a permanent, tamper-proof integration. An antenna in direct contact or embedded in the tire, however, presents difficulties. Radio frequency energy travels along the surface of the antenna. The antenna must radiate radio frequency through the surrounding elastomeric materials from which tires are typically constructed. However, such materials are usually electrically conductive and have a relatively high dielectric constant, typically 3 or greater. Conductive material in contact with an antenna tends to dissipate the radio frequency energy traveling on the antenna surface. In addition, conductive dielectric material in contact with an antenna allows radio frequency current to pass between the two adjacent feed points of the antenna, also dissipating radio frequency energy. The problem of dissipation increases with the frequency, and is particularly troublesome at or above very high frequency (130 MHz) operation. Furthermore, placement of the antenna within a dielectric material such as tire rubber causes significant changes in the resonance and impedance of the antenna, making the antenna appear longer from an electrical standpoint than when in free air. As a result, a correction must be made by either shortening the antenna or by adding an appropriate reactance.
Finally, physical problems are also encountered when embedding or otherwise placing an antenna in direct contact with a tire. For example, the antenna, typically a metallic element, must adhere to the rubber material to secure it in place. Further, the antenna material must withstand the cyclic stresses in the functioning tire.